Merge branch 'nd/diff-quiet-stat-dirty' into maint

"git diff --quiet -- pathspec1 pathspec2" sometimes did not return
correct status value.

* nd/diff-quiet-stat-dirty:
diff: do not quit early on stat-dirty files
diff.c: move diffcore_skip_stat_unmatch core logic out for reuse later

diff --cc diff.c
Simple merge

diff --cc diffcore.h
Simple merge